The fabrication of small structures, conductive or insulating, on substrates using lithography is very important, e.g., for the fabrication of microcircuits in microelectronic industry, for the fabrication of sensors, or for the manufacture of displays.
Photolithography
Photolithography is a well-known technique used to structure substrates or layers on a substrate. Patterning a layer using photolithography involves several steps. First, the layer is coated with a photosensitive layer called the photoresist. This resist is then exposed through a patterned mask with light. Depending on the tone of the resist, the resist can be developed by dissolving away the exposed areas (positive resist) or the unexposed parts of the resist can be removed (negative resist).
The patterned resist acts then as a mask to protect its substrate from etchants. Photolithography is the standard technique used to fabricate microcircuits by patterning substrates or layers on a substrate to form the various parts of integrated circuits (cf. “Semiconductor Devices, Physics and Technology” page 428, by S. M. Sze, Wiley and Sons, 1985, NY). Lithography is used worldwide by semiconductor manufacturers and is of extreme importance in technology.
Contrast and Resolution in Lithography
The contrast and the smallest features of a pattern represent two important characteristics of a lithographic technique. An ideal contrast corresponds to the entire removal of the original layer where desired without damaging the rest of this layer. The resolution is defined by the smallest achievable patterns and the edge resolution of these patterns. In practice, all lithographic techniques have a limited resolution and contrast due to imperfections in the mask and the instrumentation used to expose the resist with light and due to the process to develop the resist and the etch chemistry.
Etching the Substrate
The resist and the etch chemistry used to pattern one type of substrate must be selected carefully and must be compatible. The resist usually consists of a polymeric material with a thickness of the order of 1 micrometer or more and its function is to physically block etchants from reaching and dissolving the underlying substrate. Etching can proceed using gaseous etchants (dry etch) or using a solution containing etchants (wet etch). A dry etch is probably more controllable than a wet etch but wet etching is economically more attractive and necessary when the chemicals resulting from dry etching a substrate are not gaseous but accumulate on the substrate.
Many types of etching baths have been developed to wet etch metallic, semiconducting or insulating substrates. These baths are generally prepared by dissolving etchant species and salts in water or in a solvent. The pH, the temperature and the stirring of the bath are usually important parameters for wet etching. The substrate can be immersed into the bath for etching for a given time or, alternatively, the etch solution can be sprayed over the substrate. Some chemicals in the bath might be consumed during the etch and the bath then may be replenished or replaced.
Self-Assembled Monolayers
It is known that self-assembled monolayers (SAMs) can form on, e.g., gold or other metals. SAMs result from the spontaneous adsorption of molecules from a solution or a gas onto a substrate. SAMs-forming molecules comprise a headgroup which interacts with the substrate and the remaining part of the molecule usually acquires some order from its interaction with neighboring molecules in the monolayer. SAMs can form on a variety of surfaces depending on the headgroup-substrate chemistry. For example, alkanethiols and disulfides can form SAMs on Au, Ag, Pd and Cu, and silanes on silicon oxide, for example. The preparation, characterization and utilization of SAMs is an important field of research because these layers represent a model system to change and control the properties of surfaces. It is possible to change the wetting, lubrication and corrosion properties of a substrate covered with a SAM, for example.
Patterning SAMs
There is a number of techniques known for patterning SAMs on a surface. For most of these techniques, a first step consists in covering a substrate entirely with a SAM. In a second step, the SAM is removed in some areas using ultraviolet light, a beam of electrons, bombarded atoms, or the probe of a scanning probe microscope (scanning tunneling microscope or atomic force microscope). Another technique is called microcontact printing (MCP), and will be described in more detail below. MCP directs the formation of the SAM where desired on the substrate during a printing step. The patterned SAM resulting from these techniques serves as a resist for selectively etching the substrate.
Micro Contact Printing (MCP)
MCP employs a stamp replicated from a mold to localize the formation of a SAM on a substrate during printing. There are many constraints on the material used for forming the stamp. The stamp must be soft to ensure good contact with the substrate during printing, yet it must be mechanically stable. PDMS is a material of choice to form stamps because it fulfills in part these conditions and PDMS stamps can be inked with solutions of alkanethiols in ethanol. After evaporation of the ethanol and drying of the stamp, some alkanethiols remain in and on the stamp. The patterning of such a stamp is disclosed, e.g., in EP-B-0 784 543.
MCP works best for printing alkanethiols on Au and has been extended with more difficulties to the printing of alkanethiols on Ag, Cu and of trichlorosilanes on silicon oxide (see, e.g., Y. Xia and G. M. Whitesides (1999), Angew. Chem. Int., Ed. 37, 550–575). It is relatively simple to print alkanethiols on Au to change some of the properties of the substrate like wetting but it is considerably more difficult to employ MCP for printing and etching Au with high contrast and resolution for the following reasons.
Difficulties of Using SAMs as Resists
The ability of SAMs to protect a substrate as compared to a conventional photoresist is limited for several reasons. An ideal SAM for providing a good etch barrier to a substrate should be as thick, ordered and dense as possible. One first drawback of using SAms as resists derives from the impossibility to prepare SAMs having a thickness greater than a few nanometers. The formation of a thick SAM would require chemisorbing a long molecule but long molecules tend to coil spontaneously making the chemisorbing headgroup less accessible for adsorption onto the substrate. In addition, long molecules have difficulties forming ordered and dense monolayers, because they have many more stable conformations than short molecules and not all conformations do lead to a good order in the SAM. Forming thick and ordered SAMs is desirable, however, to prevent the diffusion of etchants through disordered regions of the SAMs. It has also been found that the etch protection provided-by SAMs formed with linear alkanethiols of the general formula HS—(CH2)n−1—CH3 increases with n>12 (thick enough barrier) but decreases again for n>20 (see, e.g., X.-M. Zhao et al., (1996), Langmuir 12, 3257–3264).
There exist several examples of etch chemistries for which the rate of dissolution of the bare substrate as compared to the dissolution of the substrate covered with a SAM is higher. Au can be etched with a solution containing cyanide and oxygen but it is not easily etched when it is covered with a monolayer of alkanethiols, for example. Other etch chemistries have been investigated for thin Au substrates protected by SAMs formed from solution or using MCP. Some etch chemistries have also been investigated for Ag and Cu substrates (Xia et al., (1995), Chem. Mater. 7, 2332–2337).
It is known that the order and protection of SAMs on rough substrates is worse than for SAMs on smooth substrates. Practically, rough substrates covered with SAMs are not well protected from corrosion or etchants and substrates like Ag and Cu are rougher than Au due to their crystalline structure. Protecting Ag and Cu substrates from etchants using SAMs is thus more problematic than for Au substrates. The roughness of a substrate increases with its thickness. Consequently, it is more difficult to protect thick substrates than thin substrates with SAMs.
The chemical composition of a substrate influences the quality of the SAMs formed on them. A metallic substrate having impurities (atoms or molecules) on its surface can prevent the adsorption of molecules which can lead to local defects in a SAM. Au contaminated by airborne sulfur-containing molecules is an example. Another example is the native oxide on Ag and Cu surfaces exposed to air as well as the presence of foreign atoms like, for example, Cr, on the surface of Au due to diffusion from an underlayer. Such metal impurities or oxides interfere with the formation of a SAM and lower the etch protection conferred to a substrate by the SAM.
The etch protection of a substrate by a SAM also depends on the conditions of formation of this SAM. Some SAM-forming molecules are very reactive and side-react with water or contaminants during the SAM formation. This can lead to defects in a monolayer as well and is, e.g., the case with trichlorosilanes which are used to form SAMs on silicon dioxide surfaces. Molecules in SAMs can be damaged by ambient ultraviolet light, and by oxygen leading to defects in monolayers. This is well known and studied for SAMs of alkanethiols on Au for which ambient ultraviolet light from lamps and oxygen from air degrade the chemisorbed sulfurs into sulfonates moieties (see, e.g., Li et al., (1992), J. Am. Chem. Soc., 114, 2428–2432). The strength of interaction of the degraded molecules with the Au substrate is weakened and the degraded molecules are soluble in etch baths. This leads to loss of molecules from the SAM, which creates defects in the monolayer through which etchants can penetrate. Exposing a substrate covered with a SAM to heat, vacuum or rinses with solvents can also lead to loss of molecules from the monolayer and to etch pits of the substrate where it is not desirable.
Additionally, patterning SAMs on a surface does suffer like conventional resists from not having an ideal contrast (see, e.g., Delamarche et al., (1998), J. Phys. Chem. B, 102, 3324–3334). During the operations of adding a monolayer onto regions of a substrate, some molecules can deposit onto the substrate at undesired locations. These molecules can retard the etch in these regions even if they do not form a complete monolayer. Inversely, if the SAM is patterned by its selective removal, molecules might also be removed from adjacent areas, which can lower the etch resistance of the monolayer in these regions.
Difficulties of Using MCP to Pattern SAMs
Some specific problems exist when MCP is used to pattern a monolayer on a substrate. Only a few types of molecules can be used for MCP because the SAM-forming molecules must be soluble in solvents compatible with stamps. It is widely accepted that molecules soluble in water or ethanol can be used for inking a stamp. Other solvents swell and distort stamps too much for being of technological interest and particularly for the fabrication of microcircuits for which patterns must be as accurate as possible. Alkanethiols which form well protective monolayers on Au are not soluble in water. This means that MCP of alkanethiols on Au is restricted to the usage of alkanethiols compatible with the stamp or the MCP technique but these thiols are not necessarily optimal in term of protection of the Au surface for etching.
The formation of a well protective SAM on Au, Ag or Cu using MCP necessitates to provide enough thiol reactants from the stamp and a printing duration long enough to form a patterned SAM as good as possible. The amount of thiols present on the stamp prior to inking and the printing duration must be well optimized to prevent the diffusion of thiols away from the print areas. It is known that preventing diffusion of the thiols over the printed substrate requires to limit the amount of thiols present on the stamp and the print duration, which for practical application prevents to form fully complete printed SAMs.
Another problem for etching a layer protected by a SAM is that etching of this layer will start at defects in the SAM and propagate in the directions of etching. This considerably enlarges initial defects in the SAM even when they are very small in dimension at the beginning of the etch.
This situation worsens when MCP is used to print on Ag and Cu because these surfaces tend to be covered with a film of oxide. Due to this oxide, a fraction of the thiols provided by the stamp do not take part in the formation of SAM but react unproductively with the metallic oxide. The conditions used to ink the stamp, the printing duration and the amount of oxide present on the substrate make it unlikely to provide enough molecules to print a complete and well protective SAM on the substrate.
Another drawback of MCP is that the amount of thiols inked onto a stamp can vary locally depending on the inking method and the pattern present on the surface of the stamp (see, e.g., Libioulle et al., (1999), Langmuir 15, 300–304). The transfer of thiols from the ink solution into the stamp depends on the interface of contact between the stamp and the solution. This interface varies with the geometry of the pattern of the stamp. This effect is well known and called the “geometric effect”. An important consequence is that during printing the local concentration of thiols and the release of thiols from the stamp is variable. This leads to the formation of SAMs onto the printed substrate with varying degree of completion and with varying etch resistance, and is thus very detrimental to the formation of patterns having structures with different sizes and shapes like it is necessary to have for designing microcircuits. One solution to this problem could be to print the entire pattern in several steps using different conditions depending on the type of subpattern being printed. This approach would have reduced yields, however, and would be costly.
Taper
It may be desirable to control the profile of etched structures in some cases. In some applications like for the formation of conductive lines for displays, it is essential to have a tapered edge of the lines and elements forming the first pattern on the substrate. The taper allows for a good and smooth coverage of the pattern by additional layers. It is well known that the edge profile usually obtained by etching a substrate having a pattern of resist is a recessed edge. This profile corresponds to a negative or sometimes curved taper and is due to an underetch of the substrate below the resist. With such a profile, it is not possible to deposit an additional layer with good coverage and without having a gas or liquid trapped in cavities. It is well known that such a problem leads to defects in the final device.
Controlling and achieving a taper is difficult and restricted to a few cases. A crystalline layer can be etched with a controlled edge profile when the etch chemistry used is anisotropic, i.e. the etch reaction depends on the crystalline orientation of the layer. This method works for crystalline substrates like Si single crystal wafers but not for amorphous substrates like glass. Another technique to achieve a taper is to add a layer on top of an existing layer, which etches faster than the underlayer. The faster etch of the top layer widens the area of the underneath layer exposed to the etch, which results in taper. This strategy, however, shows an important drawback in that it requires the costly deposition of an additional layer and this layer must also be compatible with both the underlayer and the overlayer and the metallurgy of the device in general. The additional layer must etch faster than the underlayer, what, in turn, considerably reduces the chance of having a solution for a particular case. Another possibility to achieve a taper is to employ a reactive ion etcher. In this case, the etch proceeds in the gas phase, under reduced pressure, and the patterned resist is reflowed during the etch to vary the areas of the substrate exposed to gaseous etchants. This strategy is limited to materials which can be etched in the dry phase and it is costly due to the complex instrumentation it requires.
Blocking a Gold Electrode Covered with a Monolayer
M. French and S. E. Creager in Langmuir 1998, 14, 2129–2133, “Enhanced Barrier Properties of Alkanethiol-Coated Gold
Electrodes by 1-Octanol in Solution”, describe the fact that the barrier properties of alkanethiol-coated gold electrodes are enhanced when the surfactant 1-octanol is present in the aqueous electrolyte solution surrounding the electrode: oxidation on the electrode surface of ferrocyanide from solution is largely suppressed and the oxidation of the gold electrode is shifted towards a more positive potential (vs the Ag/AgCl reference electrode) in an octanol-saturated buffer at pH 5. The enhanced barrier properties are thought to be caused by a thin layer of octanol atop the alkanethiol monolayer. The octanol fills in defects in the alkanethiol monolayer and increases the overall thickness of the barrier layer.
It is clear from the above mentioned reasons that SAMs cannot be formed or patterned on a substrate as to form an ideal resist. It is also clear that many parameters must be taken into account for producing high quality patterns with a surface protected by SAMs. It is clear as well that applications typical of lithography necessitate high quality patterns.